The present invention relates to optical prisms which when rotated cause a rotation of an image being projected through the prism; and more particularly to such prisms for use in an image projection system for a microfilm viewer/printer.
Microfilm is commonly used for archival storage of large volumes of documents. Viewing equipment is necessary for a user to read documents stored on microfilm and printers are required to make an enlarged paper copy of such documents. Consequently, machines which perform both functions, commonly referred to as microfilm viewer/printers, are commercially available and widespread. Such microfilm viewer/printers have one mode in which the microfilmed image is projected onto a screen for observation by the user. If the user desires a paper copy of the projected image, a button on the machine is depressed and the image is directed toward a electro-photographic copying apparatus inside the machine.
The recording medium is referred to herein generically as microfilm, a term which includes a long strip of a photographic transparency much like motion picture film in which the different documents are recorded at sites along the length of the strip. Such long strips of microfilm are stored on reels. Another microfilm medium, commonly referred to as microfiche, is a flat, rectangular photographic transparency containing rows and columns of sites for the microfilm images. In both forms, the microfilm site is rectangular in which the length of the site is longer than the width in a proportion that conforms to the proportions of standard sizes of printed documents.
Most documents have lines of text extending across the smaller dimension of the page. However, computer printouts commonly have lines of text extending along the longer dimension of the page. Thus, when both types of documents are microfilmed, the lines of text in a image from a computer printout are oriented orthogonally on the medium to the orientation of the lines of text from standard documents.
In order to accommodate the display of both types of documents, microfilm viewers previously provided a film holder which could be rotated into two orthogonal viewing positions. Thus the viewer could orient the medium so that the lines of text appeared horizontally on the viewing screen. In one orientation, the longer dimension of the paper was projected vertically on the screen for viewing standard documents and in the other orientation, the longer dimension was projected horizontally for reading computer printouts.
However, the printing mechanism for creating paper copies was often mounted in the viewer/printer in a fixed orientation to the screen. One type of view/printer was designed to accommodate computer printouts by orienting the printer to copy documents being displayed with the longer dimension projected horizontally on the screen. Thus in order to print standard documents recorded on microfilm, the user had to rotate the film holder to project the lines of text vertically on the screen before pressing the print button. Alternatively, the view/printer could be designed with the printing mechanism oriented for standard documents. While this system facilitates processing standard documents recorded on microfilm, the film holder still must be rotated between viewing and printing computer printouts.
Although motorized mechanisms could be developed for automatically rotating the film holder between the viewing and printing modes, such mechanisms increase the size of the microfilm apparatus. The motorized mechanisms also present a safety hazard as the user is typically holding onto the film holder in order to advance the microfilm from one image to the next. Thus, the user's hands could be struck or pinched by the automatically rotating film holder.
As an alternative to rotating the film holder, previous microfilm viewer/printers utilized a Dove prism, as shown in FIG. 1, through which the light was projected from the microfilm medium to either the screen or the printing apparatus. When this type of prism is rotated, the projected image also rotates even though the optical object remains stationary. By rotating the Dove prism between the viewing and printing modes, the image of the document is rotated into one orientation for reading by the viewer and into another orientation to fit onto a sheet of paper within the printing apparatus. Furthermore, since the Dove prism and its associated components are inside the viewer/printer, the user can not come into contact with the rotating mechanism.
FIG. 2 shows a reversion prism which exhibits similar image rotation properties as a Dove prism. The reversion prism is formed by two pieces of optical glass for ease of fabrication. The pieces abut one another so that light passing through the prism is not refracted or reflected at their interface. One of the reflective surfaces 8 of a reversion prism is coated with silver or other reflecting material to reflect the light back into the prism. The axial light ray 9 enters and exits the reversion prism on the rotation axis of the prism.
The use of Dove prisms in microfilm viewers/printers present several problems. First, the prisms change the effective focal length of the lens system as the light rays diverge less in the prism than in air. Therefore, if a manufacturer produces two models of a microfilm viewer/printer, with and without the rotating prism but otherwise the same, different focal length lenses are required in each model to achieve the same magnification. This not only increases manufacturing costs, but requires the stocking of both types of lenses for maintenance and repair purposes. Secondly, the Dove prism has a relatively long length affecting the ability to reduce the size of the microfilm viewer/printer to make it as compact as possible. It is desirable in today's marketplace to have a compact device which can fit on a table top and be transported easily.